I'm pretty sure (based on cards issued to me) that the first few digits identify the bank, and for at least one bank, the last digits are issued predominantly sequentially.

I had something similar happen to me reasonably recently. I called a friend on my cell phone and another, completely unrelated friend answered. It turned out that the last 7 digits of both phone numbers were equal (though the area codes were different). I know that phone numbers aren't uniformly distributed, but I still think this would be a pretty rare occurrence. Dave

My father purchased a new Buick in 1950 & found that one of his old keys that came with a previous GM car worked in the new Buick. When I was a kid, I thought that this story was highly improbable, until I talked with someone who had worked as a "repo man" during his college years. He told me that he needed only a small pocketful of keys to open virtually any (U.S.) car, because the (U.S.) car companies at that time didn't have very many distinct locks. I would have thought that having only this small number of lock/key combinations had stopped after car thefts skyrocketed in the 1970's & the car companies were forced to put in better security measures. My current car is a relatively ubiquitous car in a very ubiquitous color, and I have gone so far as to attempt to open the wrong car with my (electronic) key on numerous occasions. --- Re keys: I read recently where someone (at MIT??) had been successful in duplicating a real key from a surreptitious video of the original (traditional mechanical) key in someone's hand. I guess it was relatively easy to figure out the lock manufacturer & then the vertical grooves & then the teeth. Apparently, the number of distinct lengths of teeth isn't very large. At 12:04 PM 6/24/2009, Hilarie Orman wrote:

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I called a friend on my cell phone and another, completely unrelated friend answered. It turned out that the last 7 digits of both phone numbers were equal

It's not all that unusual, but it might indicate that you have too many friends and call them too often.

Anyone in your "clique" will see the same "collision".

I came out of a meeting at night, started driving home, and was annoyed that the lights on the radio panel were burned out. No, that wasn't it, there were too many lights. Or something. A few blocks later I began to wonder why in the world my husband had put a folded towel on the passenger seat. Whatever was on his mind? Unless ... this wasn't my car. No, it wasn't. I started driving back to where I had parked, worried that the car's owner had by now discovered the loss and called the police. Or, should I think seriously about whether or not this was a better car? It was certainly cleaner than mine. I found my car where I had left it, parked in the space next to it, locked the borrowed car, and drove away.

How unusual is that? Well, the car is a very common model, so the probability of parking next to one it pretty high. The keys aren't drawn from a terribly large space, and older cars become more accepting of alternate keys. Now I am more careful about checking for my bumper stickers before getting in the car. I think "Ultimate Frisbee" and "Soaring" provide enought bits of entropy for all practical purposes.